Do diesel vehicles cause poor air quality?

Poor air quality in the UK is mainly caused by diesel vehicle emissions. Although improvements have been made in the latest generation of diesel vehicles, testing under real driving conditions showed that their emissions of nitrogen oxides are still six times greater than the EU limit value. Diesel engines also emit airborne particulate matter which is thought to contribute to more than 5% of all UK deaths. A new emissions testing regime will begin in 2019, but there is long way to go before health-based air quality objectives will be met.

Air pollution is a major issue in the UK. Exposure to small airborne particles (known as PM2.5) contributes to about 6% of all deaths in England, with additional deaths and ill health caused by exposure to nitrogen dioxide. Approximately two thirds of local authorities have declared air quality management areas, mainly for high NO2 concentrations due to emissions of nitrogen oxides (NOx) from road traffic. Furthermore, the EU annual mean NO2 limit value is exceeded in 38 out of the 43 air quality monitoring zones in the UK. In December 2015 the Government published its national air quality plan “Tackling nitrogen dioxide in out towns and cities”. This states that “on average transport is responsible for 80% of NOx emissions at the roadside in areas where we need to act to reduce levels”. Diesel vehicles are the largest source of NOx. Whilst heavy duty vehicles such as lorries and buses tend to be diesel fuelled, and have been for many years, the number of diesel light duty vehicles (cars and vans) has been increasing. These vehicles have much higher NOx emissions than their petrol equivalents, but despite this they were promoted by successive governments through more favourable taxation due to their lower carbon dioxide emissions.

NOx and particulate matter (PM) are commonly produced during combustion processes. NOx is generally considered to be a combination of nitric oxide (NO) and nitrogen dioxide (NO2). Most of the NOx emissions from diesel vehicles are as NO, which is rapidly oxidised in air to NO2. PM is a heterogeneous mixture of pollutants emitted from many sources, including combustion. PM is also formed in the atmosphere from air pollutants such as NOx, sulphur oxides and organic compounds.
Generally there is a trade-off in emissions between fuel efficiency (and hence CO2 emissions), PM and NOx. For a given fuel efficiency or CO2 emission an engine can be calibrated to have either low NOx or low PM emissions, not both. For low CO2 emissions there are likely to be high NOx emissions. Consequently, the emission limits for NOx are more lenient for diesel than for petrol cars. However, there are PM emission limits for diesel vehicles but not for most petrol vehicles, as generally petrol cars have virtually no PM emissions.

New vehicles must pass a series of type approval tests. Car emissions are tested in a laboratory on a rolling road as the car is driven over a standard test cycle. Petrol cars achieve the emission limits in the laboratory by a wide margin; diesel cars less so. However, remote sensing and portable emission measurement systems have shown that emissions from diesel cars are much higher when driven on the road than under laboratory conditions. This is not the case for petrol cars, which achieve similar results in the laboratory and on the road and outperform diesel in both environments. There has been little improvement in NOx emissions from diesel vehicles (both heavy and light duty) until very recently. For example, the latest generation of diesel vehicles have been shown to have lower on-road NOx emissions than earlier generations; with Euro 6 cars, for example, having about half the emissions of Euro 5 cars.

A car emissions testing centre in Turin, Italy, in 2014. New diesel cars will soon have to pass more stringent emissions tests. Credit: MikeDotta/Shutterstock

This is not a newly identified problem. As far back as 1993 the Quality of Urban Air Review Group (of which I was a member) concluded that “…unless some improvements in the emissions from diesel vehicles can be achieved, there must be considerable concern over any increase in the proportion of diesel vehicles on our urban streets as their impact on urban air quality is undoubtedly quite serious” [page 69 in (1)]. The diesel share of the new car market has increased from 20% to almost 50% since that report was published. In some other European countries, the share is currently around 70%. Greece used to have the lowest share in the EU until 2011, when a ban on diesel cars in the country’s two largest cities, Athens and Thessaloniki, was removed. In just four years diesel car sales increased from 4% to over 60%.

New vehicles must pass a series of type approval tests. Car emissions are tested in a laboratory on a rolling road as the car is driven over a standard test cycle. Petrol cars achieve the emission limits in the laboratory by a wide margin; diesel cars less so. However, remote sensing and portable emission measurement systems have shown that emissions from diesel cars are much higher when driven on the road than under laboratory conditions. This is not the case for petrol cars, which achieve similar results in the laboratory and on the road and outperform diesel in both environments. There has been little improvement in NOx emissions from diesel vehicles (both heavy and light duty) until very recently. For example, the latest generation of diesel vehicles have been shown to have lower on-road NOx emissions than earlier generations; with Euro 6 cars, for example, having about half the emissions of Euro 5 cars.

This is not a newly identified problem. As far back as 1993 the Quality of Urban Air Review Group (of which I was a member) concluded that “…unless some improvements in the emissions from diesel vehicles can be achieved, there must be considerable concern over any increase in the proportion of diesel vehicles on our urban streets as their impact on urban air quality is undoubtedly quite serious” [page 69 in (1)]. The diesel share of the new car market has increased from 20% to almost 50% since that report was published. In some other European countries, the share is currently around 70%. Greece used to have the lowest share in the EU until 2011, when a ban on diesel cars in the country’s two largest cities, Athens and Thessaloniki, was removed. In just four years diesel car sales increased from 4% to over 60%.

For many years France had one of the highest proportions of diesel cars, reaching a peak of over 75% in 2008. The French car manufacturers, particularly the PSA (Peugeot and Citroen) group, were among the early promoters of diesel cars. In the early 1980s, diesel cars were reliable, durable, and fuel efficient, but unrefined to drive. Two technical advances led to diesel cars becoming as attractive to motorists as petrol cars; turbo-charging and direct injection diesel, and from 2001 their fuel economy benefits led the UK Government to provide tax incentives through a CO2 based vehicle excise duty.

New post 2015 (Euro 6) petrol cars have much higher carbon monoxide and hydrocarbon emissions than an equivalent generation of diesel car, and slightly higher CO2 emissions. On the other hand, the NOx and PM emission from diesel cars are much higher. The political pressure to reduce CO2 emissions led to some petrol engines becoming more similar to diesel engines, and as a result their NOx and, particularly, PM emissions are higher. This new type of engines are known as direct injection gasoline or GDI engines and are subject to a PM limit for the first time.

The NOx emission limit is more lenient than for petrol because of the difficulty in reducing NOx emissions from diesel cars; diesel cars are permitted to emit 33% more NOx than petrol cars. This is, however, only one element of the diesel NOx problem. While on-road NOx emissions from petrol cars meet the limit value, those from diesel cars are many times higher. The vehicle emissions testing programme (2) found that on average, Euro 6 diesel cars emitted more than six times the limit value when driven on the road.

The other problem with diesel cars is that they have much higher primary (direct) NO2 emissions than petrol cars. Primary NO2 emissions have a large influence on NO2 concentrations close to busy roads. Researchers at King’s College London have analysed trends in ambient NO2 concentrations (3) at roadside locations in London over the periods 2005-2009 and 2010-2014. Between 2005 and 2009 there was an increase in NO2 concentrations. However, over the second period there was an average reduction of NO2 of nearly 10% as well as a large reduction in PM2.5 (28%) and black carbon (11%) (4). The trends were not fully explained by lower levels of traffic and therefore it is likely that the reduction was due to a decrease in primary NO2 emissions. Despite the general downward trend in roadside NO2 concentrations, this was not observed at all roadside monitoring sites.

In relation to high NOx emissions from diesel cars, an analysis (5) of the results of the real‐world emission testing programmes was undertaken by several European governments in response to the Volkswagen emissions scandal, and showed that there is a huge variation in NOx emissions when measured as the vehicle is driven on the road, with ironically Volkswagen cars performing the best, but the emissions were still about twice the laboratory-based limit value. The worst performing vehicles, from Renault, Nissan, Fiat and Suzuki, had NOx emissions more than 14 times the limit value. These high real world emissions are caused by a range of strategies that car manufacturers have adopted for managing the pollution control system. These include both the engine management system recognising the laboratory test cycle, and switching off the emission controls after 22 minutes (the test cycle lasts about 20 minutes). Other manufacturers switch off the system when the ambient temperature is too cold or too hot, or when the engine is under load (i.e. when emissions are high). The average temperature in southern England is in the range of 12 to 15 °C, yet two car manufacturers switch their pollution control systems off when the ambient temperature is less than 17 °C.

A new real driving emission (RDE) test is to be introduced for new car types from September 2017, and for all new cars from September 2019. This will limit real driving emissions to 2.1 times greater than the laboratory based limit value. A second stage is to be introduced from January 2020/2021 to reduce the RDE emissions to 1.5 times the laboratory limit. To meet these new requirements, manufacturers may need to combine emission abatement technologies, such as selective catalytic reduction and lean NOx traps, to ensure they work effectively over the whole operating range of a diesel engine. This may be a challenge for small diesel cars, especially if much larger on-board storage tanks for AdBlue (6) are needed, as there is little spare space.

Finally, are the benefits of diesel cars regarding lower CO2 emissions real? Similar to the NOx emissions, CO2 emissions measured in laboratory tests are also much lower than the real driving emissions. Comparison of the official CO2 emissions with large databases of real fuel consumption data (7) suggests that the difference between the laboratory and real world CO2 emissions has grown since 2001, when the UK first gave an incentive for low CO2 emitting cars. In 2001, real world CO2 emissions were 8% higher than laboratory measurements, but this had risen to 40% in 2015. The greatest differences between real world and laboratory emissions were for hybrid vehicles (but the sample size was much smaller), followed by diesel and then petrol cars. The official CO2 emissions data show similar emissions from petrol and diesel cars; given that real world CO2 emissions from diesel cars are greater than those measured in the laboratory, there may in reality be little difference between the two car types. However, diesel cars tend to be larger than petrol cars, and comparing average emissions could be considered unfair.

In summary, emissions of air pollutants from petrol cars are well controlled. Controlling NOx emissions from diesel vehicles has evolved more recently and for many years has not been effective. There is some evidence that the most recent diesel vehicles have lower emissions than earlier generations and this should accelerate with the introduction of RDE tests for cars. There will, however, remain some uncertainty until these vehicles are in use and independently tested. In many towns and cities there remains a long way to go to ensure that health based air quality standards are achieved.

References and notes

Diesel vehicle emissions and urban air quality, Second report of the Quality of Urban Air Group, 1993, available at https://uk-air.defra.gov.uk/assets/documents/reports/empire/quarg/quarg_94.pdf.

Black carbon is formed by the incomplete combustion of carbon containing fuels.

Routes to Clean Air, Institute of Air Quality Management, Bristol, 11-12 October 2016, see http://iaqm.co.uk/event/routes-to-clean-air-2016/

AdBlue is the urea additive used to form ammonia on-board, which is used to create a reducing environment to enable the catalyst to remove the NOx in the vehicle exhaust.

International Council for Clean Transportation, From laboratory to road: a 2016 update of official and ‘real-world’ fuel consumption and CO2 values for passenger cars in Europe, 2016, available at http://www.theicct.org/sites/default/files/publications/ICCT_LaboratoryToRoad_2016.pdf.